Sintered ferrite magnets having magnetoplumbite-type (M-type) structures are used in various applications including motors, rotors of electric generators, etc. Sintered ferrite magnets having higher magnetic properties are recently required for the purpose of reduction in size and weight of motors for automobiles and increase in efficiency of motors for electric appliances. Particularly, sintered ferrite magnets used for rotors for automobiles are required to have not only high Br for reduction in size and weight, but also high HcJ to be resistant to demagnetization, when it is subjected to a demagnetization field generated by thinning them.
M-type sintered ferrite magnets such as Sr ferrite, Ba ferrite, etc. have conventionally been produced by the following steps. An iron oxide and a Sr or Ba carbonate, etc. are mixed and calcined to produce calcined clinker by a ferritization reaction. The calcined clinker is coarsely pulverized, and a predetermined amount of the resultant coarse powder is charged into a fine pulverizer, together with SiO2, SrCO3, CaCO3, etc. for controlling sintering behavior, and Al2O3 or Cr2O3 for controlling Hcj, if necessary, and wet fine pulverization is conducted with water as a medium to an average particle size of about 0.5 μm. The resultant slurry containing fine ferrite particles is molded in a magnetic field, dried and then sintered. The resultant sintered body is machined to a predetermined shape to provide a sintered ferrite magnet.
In the above production, when fine powder particles in the slurry obtained by fine pulverization in a wet state have an average particle size of less than 0.7 μm, the dewatering of a green body in a slurry-molding step in a magnetic field takes remarkably long time, resulting in drastic decrease in the number of moldings formed per unit time (molding efficiency). This problem makes sintered ferrite magnets more expensive. If a slurry containing fine powder having an average particle size of 0.7 μm or more is molded in a magnetic field, increase in the average particle size remarkably improves molding efficiency but drastically deteriorates the magnetic properties of a sintered ferrite magnet. This phenomenon is clear from the later-described behavior shown in FIGS. 12(a) and 12(b). Also, because fine pulverization results in low wet-molding efficiency, it is necessary to use magnetic material powder having a relatively large average particle size.
Japanese Patent 3,181,559 discloses a sintered ferrite magnet comprising hexagonal ferrite as a main phase, and having a composition represented by the general formula: Ca1-xRx(Fe12-yMy)zO19, wherein R is at least one element selected from the group consisting of rare earth elements (including Y) and Bi, La being indispensable, M is Co and/or Ni, and x, y and z meet the conditions of 0.2≦x≦0.8, 0.2≦y≦1.0, and 0.5≦z≦1.2. In FIG. 2 in Example 2 of Japanese Patent 3,181,559, Sample No. 2 sintered with 20% of O2 has Br of 4.4 kG (440 mT) and HcJ of 3.93 kOe (313 kA/m). Although it is described that the fine pulverization of Sample No. 2 was conducted for 40 hours in xylene with a ball mill, the average particle size of the resultant fine powder is not described. Japanese Patent 3,181,559 reports in Paragraph 18 and Example 6 that the sintered ferrite magnet described therein has about 2% higher saturation magnetization (4πIs) and about 10% higher anisotropic magnetic field (HA) than those of SrM. This potential of 4πIs and HA may make it possible to achieve Br of 4.6 kG (460 mT) or more and about 10-% increase in the maximum of HcJ, which would not be achieved by SrM. In view of this, Br and HcJ of Sample No. 2 are lower than the inherent potential of magnetic properties, leaving room for improvement. Although Japanese Patent 3,181,559 describes a composition encompassing the composition range of the sintered ferrite magnet of the present invention, it neither describes nor suggests the addition of a small amount of Ba to remarkably improve powder characteristics and magnetic properties.
JP11-97225A discloses an anisotropic sintered magnet comprising hexagonal, magnetoplumbite-type ferrite as a main phase, which has a composition represented by the general formula of Ba1-xRx(Fe12-yMy)zO19, wherein R is at least one selected from the group consisting of rare earth elements (including Y) and Bi, M is Co or Co and Zn, 0.04≦x≦0.9, 0.3≦y≦0.8, and 0.7≦z≦1.2. Although Table 1 lists the composition of each calcined sample corresponding to the above anisotropic sintered magnet, each composition is outside the composition range of the present invention because of too much Ba or too little Ca. Further, FIG. 1 shows low Br and HcJ.
WO 2005/027153A discloses a sintered ferrite magnet having an M-type ferrite structure, and comprising an A element which is Sr or Sr and Ba, an R element which is at least one rare earth element including Y (indispensably including La), Ca, Fe and Co as indispensable elements, the sintered ferrite magnet being produced by pulverizing, molding and sintering an oxide-type, magnetic material. The basic composition of the oxide-type, magnetic material is represented by the following general formula (1):A1-x-yCaxRyFe2n-zCozO19 (by atomic ratio)  (1), andthe basic composition of the sintered ferrite magnet is represented by the following general formula (2):A1-x-y+aCax+bRy+cFe2n-zCoz+dO19 (by atomic ratio)  (2).In the general formulae (1) and (2), x, y, z and n respectively represent the amounts of Ca, the R element and Co and a molar ratio in the oxide-type, magnetic material, and a, b, c and d respectively represent the amounts of the A element, Ca, the R element and Co added to the oxide-type, magnetic material in the pulverizing step, each meeting the following conditions: 0.03≦x≦0.4, 0.1≦y≦0.6, 0≦z≦0.4, 4≦n≦10, x+y<1, 0.03≦x+b≦0.4, 0.1≦y+c≦0.6, 0.1≦z+d≦0.4, 0.50≦[(1−x−y+a)/(1−y+a+b)]≦0.97, 1.1≦(y+c)/(z+d)≦1.8, 1.0≦(y+c)/x≦20, and 0.1≦x/(z+d)≦1.2. However, this sintered ferrite magnet is not included in the composition range of the present invention, because it indispensably contains Sr, and because the amount of Sr or (Sr+Ba) is more than the amount of Ca. Although the sintered ferrite magnet described in WO 2005/027153A has high magnetic properties, further improvement in magnetic properties is desired because of increasingly higher demand of performance by users.